https://llvm.org/bugs/show_bug.cgi?id=26702
Bug ID: 26702
Summary: [InstCombine] bitcasts hinder simple vector logic ops
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
#include <xmmintrin.h>
// There are no 'less-than' or 'less-than-or-equal' instructions.
// So use 'not greater-than -1'.
__m128i is_negative(__m128i x) {
__m128i ones = _mm_set1_epi32(-1);
__m128i isposorzero = _mm_cmpgt_epi32(x, ones); // x > -1
__m128i isneg = _mm_xor_si128(isposorzero, ones); // not pos or zero
return isneg;
}
$ ./clang pcmp.c -O1 -mavx -S -o -
...
vpsrad $31, %xmm0, %xmm0
vpcmpeqd %xmm1, %xmm1, %xmm1
vpxor %xmm1, %xmm0, %xmm0
vpxor %xmm1, %xmm0, %xmm0
retq
------------------------------------------------------------------------------
The first problem seems to be in instcombine:
define <2 x i64> @is_negative(<2 x i64> %x) #0 {
entry:
%0 = bitcast <2 x i64> %x to <4 x i32>
%.lobit = ashr <4 x i32> %0, <i32 31, i32 31, i32 31, i32 31>
%.lobit.not = xor <4 x i32> %.lobit, <i32 -1, i32 -1, i32 -1, i32 -1>
%1 = bitcast <4 x i32> %.lobit.not to <2 x i64>
%xor.i = xor <2 x i64> %1, <i64 -1, i64 -1>
ret <2 x i64> %xor.i
}
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